java: support OpenJDK 10+

Starting OpenJDK 10, the 'javah' utility is no more provided [1], and
its functionality is provided by 'javac' itself.  Hence, do not error
out on missing 'javah', and store whether it was found; in case it is
not, then:
1) assume 'javac' has the -h parameter to generate the C header, and
   make use of it
2) tell the buildsystem that com_redhat_et_libguestfs_GuestFS.h depends
   on the JAR, since the the header is generated at the step (1)

[1] https://bugs.java.com/view_bug.do?bug_id=JDK-8182758

(cherry picked from commit 662dc5d0bf)

java/Makefile.am

@@ -77,6 +77,9 @@ if HAVE_JAVA
 
 JAVAC_FLAGS = $(EXTRA_JAVAC_FLAGS) -encoding utf-8
 JAVADOC_FLAGS = -encoding utf-8
+if !HAVE_JAVAH
+JAVAC_FLAGS += -h .
+endif
 
 # Java source.
 
@@ -125,10 +128,18 @@ libguestfs_jni_la_LDFLAGS = -version-info $(JNI_VERSION_INFO) -shared
 
 BUILT_SOURCES = com_redhat_et_libguestfs_GuestFS.h
 
+if HAVE_JAVAH
+
 com_redhat_et_libguestfs_GuestFS.h: $(java_prefix)/GuestFS.class
 	rm -f $@
 	$(JAVAH) -classpath $(srcdir):. com.redhat.et.libguestfs.GuestFS
 
+else
+
+com_redhat_et_libguestfs_GuestFS.h: libguestfs-${VERSION}.jar
+
+endif
+
 # Documentation.
 
 noinst_SCRIPTS = doc-stamp